December 2016 - Nektar-users

Using PhysDeriv() for gradient-based source terms
by Kilian Lackhove 12 Jan '17

12 Jan '17

Dear List, i am trying to implement a source term based on the gradient of an auxiliary field in the APESolver. My problem is quite general, so i in terms of the simpler ADR solver, a minimal example would look like this: du / dt + v * grad(u) = c * grad(v) with u being a transported variable, v the advection velocity and c a constant. The problem is the gradient on the right hand side. I tried computing it by using ExpList::PhysDeriv(), but since the projection is not C1-continuous, this fails at the element edges. I attached an example plot to illustrate the problem. p0/rho0 uses the left axis, the gradient the right hand side one. Do you have any suggestions on how to handle the gradient term? Is there some other method i should use? Cheers, Kilian -- Kilian Lackhove, M.Sc. Fachgebiet für Energie- und Kraftwerkstechnik L1|08 114 Technische Universität Darmstadt Jovanka-Bontschits-Straße 2 D-64287 Darmstadt Germany Tel.: +49 6151 16 - 28915 Fax: +49 6151 16 - 6555 e-mail: lackhove(a)ekt.tu-darmstadt.de

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Compressible flow solver (Nektar++)
by devabratasahoo＠campus.technion.ac.il 27 Dec '16

27 Dec '16

Hi Fabian, I wanted to know could you come up with any solution of how to use the compressible flow solver in Nektar? I could come up with a better mesh for my case and I am able run the compressible flow solver with P-4, but residuals are still good at all. Below is the mach contour obtained at time 1sec simulated at a time step of 0.0001. [cid:797fa64e-55db-4abc-aa5b-4e6c83a1a06f] Furthermore, if I try to solve it using CFL tool, it terminates for all CFL values more than 0.1(which is not good). What about your status? Can anyone help me out of this situation? Regards, Dev

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Need book exercise solutions
by Prashant Tarey 27 Dec '16

27 Dec '16

Hello All Nice to see so many Nektar++ users. Actually I am trying to work on the exercise problems given in the book "Spectral/hp Element Methods for Computational Fluid Dynamics: Second Edition- by George Karniadakis and Spencer J. Sherwin". And the solutions were given on Prof Sherwin's website, which is not opening since last 4-5 days. If someone has the copy of solutions for Chapter 3 and 4, please mail me at tarey.prashant(a)gmai.com Thanks!! Prashant

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Nonphysical terminated simulation - CompressibleFlowSolver
by Selbach, Fabian 16 Dec '16

16 Dec '16

Dear users, I hope you can help one more time. I got nonphysical oscillations during my airfoil-simulation. At that time my simulations terminate with the message: "NaN found during time integration". For more information of the error and parameters see pictures below. I tried to solve this by a higher polynomial degree up to p=7 (this generates 1.5 Mio grid-points here for a 2D simulation), but the simulation terminates still. The time step itself is computed by a fixed CFL=0.9 condition. Maybe this is the wrong way, because this condition is not sufficient? So please can somebody give me a hint about that? Do I have to use another kind of grid, because the resolution of the boundary at the peak of the airfoil is not high enough or the grid lines have no be orthogonal to the surface? I am already using the spherigons-tool and tried to get a smooth boundary of my airfoil. I am using the CompressibleFlowSolver on a parallel run up to 96 CPUs by solving the Euler-Equations (EulerCFE) and the Navier-Stokes-Equations (NavierStokesCFE). The simulation terminates for both. By the way what is the difference between EulerCFE and EulerADCFE (there are no information about it anywhere)? I tried to run the example NACA0012 of the paper "Nektar++: An open-source spectral/hp element framework". The conditions use the EulerADCFE, but by running this example the simulation does not terminate, but also does not change (values of every variable in the whole domain are constant over the time - no changes?). Thanks for any help! Best regards Fabian Visualization of the grid at a polynomial degree of p=2 just before terminating: [cid:71608e01-1455-4c20-abbb-9508cb48f805] With auto-scale: [cid:bba140ce-246a-4689-a539-79ff287727b0] Same solution with scaling by user: [cid:a6bda960-d10e-4f91-a833-2e5537f8758c] Please take into account the scaling above. Error and conditions for example p=2: [cid:0ebd995c-1bae-4c43-9d08-91fe685022bf] [cid:300abb94-0f24-4dde-8cda-f67f326ea4f8] [cid:a43f5fef-3537-4385-a2d9-2f49afbcf006] Visualization of the grid at a polynomial degree of p=7 just before terminating: [cid:4d0f11c2-718a-4f85-860f-e5c90684699e] (a few time steps after that the simulation terminates and the values go against inf...)

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High order outflow boundary conditions
by Sherwin, Spencer J 16 Dec '16

16 Dec '16

Hi, Just a quick note to highlight that we have now merged into Master a branch for high order outflow boundary condition on the incompressible Navier Stokes problems. This merge now supports the “convective like” outflow conditions proposed by Steve Dong (JCP 2015). It also allows for a slightly modified version of the velocity correction scheme where we formulate the pressure as a weak problem rather than starting from the strong form of the pressure Poisson problem. This is also consistent with the work of Guermond & Shen on these scheme. I have tried to motivate both of these modifications by restructuring the description of the method in the user guide. Cheers, Spencer. Spencer Sherwin McLaren Racing/Royal Academy of Engineering Research Chair, Professor of Computational Fluid Mechanics, Department of Aeronautics, Imperial College London South Kensington Campus London SW7 2AZ s.sherwin(a)imperial.ac.uk<mailto:s.sherwin@imperial.ac.uk> +44 (0) 20 759 45052

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Fld2Tecplot error
by 丁老师 16 Dec '16

16 Dec '16

Dear Nektar group: ./Fld2Tecplot regularizedliddriven.xml regularizedliddriven.fld Fatal : Level 0 assertion violation EquationSystem 'VelocityCorrectionScheme' is not defined. Ensure equation name is correct and module is compiled. how to resolve this problem. Regards Your Sincerely Ding

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How to monitor the divergence of flow field during iteration
by ztdepyahoo＠163.com 16 Dec '16

16 Dec '16

Dear friends； I want to monitor the divergence of flow field during each time step. how to obtain this. ReGards

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Kinetic energy spectra
by Manuel F. Mejía De Alba 14 Dec '16

14 Dec '16

Dear Nektar users and team, I'm working on the correct "guess" to parameters SVVCutoffRatio and SVVDiffCoeff of the model SVV LES (SpectralVanishingViscosity). I trying to determine the influence of that values on the Kinetic energy spectra. So, I need to get the values of the Kinetic energy as a function of the k (where k = 1:NUMMODES). Using the filter "Modal energy" (user-guide 3.4.9) it is possible to obtain the integral over all the velocity not with of the modes k. It is possible but just with one-dimensional homogeneous expansion and I wanna plot the data on a general model. May anyone gives me a hand with this, any help is appreciated. Thanks in advance, --- Manuel Felipe Mejía De Alba.

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Re: [Nektar-users] Nektar++ questions
by Sherwin, Spencer J 11 Dec '16

11 Dec '16

Dear Hans, Sorry for being so slow in getting back to you. Things are just been a bit busy recently. I am new to Nektar++ and I would like to ask you some questions which I couldn't find in the documentation. - Plotting streamlines: I have been trying to plot streamlines with Paraview; any additional output option must be set in the .xml file? I do not believe so since if yo have the velocity field then the plotting package should be able to work this out. - Adding tracers: is there the possibility to add tracers to the simulations? That is, a way to track particles or groups of them throughout the simulation You should be able to add a passive scalar to the incompressible Navier-Stokes solver. I believe Douglas added this feature recently and am cc’ing him to confirm it is now in the master release (note this is different to the release on the web page which is slightly older). - Probes: is there a way to put probes? i.e. to output the change of certain variables at a particular computational domain's point. Yep we call these history points. Details of this should be in the user-guide. - Iteration curves: is there a way to plot the residuals (velocity's, for example) vs iterations? How to output this? Not quite sure what you require here but if you run with the -v option you can see the number of iterations per time step. - SFD: I would need to have a look at the source where this method is defined but the search option of the doxygen-generated documentation does not work. Which file should I have a look at? You should have a look at the Driver files. There are in the library library/SolverUtils/DriverSteadyState.cpp Also it would be good if you can send these request to the nektar-user list since you may then get more helpful responses from others. Cheers, Spencer. Spencer Sherwin McLaren Racing/Royal Academy of Engineering Research Chair, Professor of Computational Fluid Mechanics, Department of Aeronautics, Imperial College London South Kensington Campus London SW7 2AZ s.sherwin(a)imperial.ac.uk<mailto:s.sherwin@imperial.ac.uk> +44 (0) 20 759 45052

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FieldConvert output fields on GLL points
by F Mellibovsky 08 Dec '16

08 Dec '16

Dear all, Is there some easy way in FieldConvert to write a vtu file with all fields computed on the GLL points instead of on equispaced element sub-meshes? For wall-y+ calculation it is the first GLL point away from the surface that should be checked, rather than the element height, which I think is given. Or when assessing if near-wall gradients are well resolved, it would be better to have fields plotted on a mesh that is representative of the modal resolution used. Thanks.

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